End mills are the primary cutting tools used in a vertical
mill such as the mini-mills we are concerned with. Like drills, they come in a wide
variety of styles and quality levels. The most common ones you will need for getting
started are 2-flute and 4-flute end mils. You can buy them individually as needed, or in
sets.

Here's a picture showing a variety of end mills:

The gold-colored mills have a very thin Titanium Nitrate
(TiN, not tin) coating which helps to prevent chips from welding to the mill surface.
The ball end mill has a round tip, for milling grooves with a semi-circular
cross-section such as you might use for a bearing race. It is an example of some of the
more specialized types of mill available.

Mills are also available in sets of 10 or 20 such as shown
below. Import quality sets on sale range in price from around $20 for a set of 10 to
around $50-$60 for a set of 20 TiN coated single-end mills.

If you are just getting started, a set such as that on the
left would be a good set to go with. You might want to also buy a few individual 2-flute
mills just to compare the difference.

Shank Size

The shank is the smooth portion of the mill shaft above (or
between, in the case of a double-ended mill) the cutting edges. Mills of different
diameters within a range have common shank sizes but shank sizes vary among larger and
smaller mills. For example, a 5/16" and a 3/8" mill both have a
3/8" diameter shank, while 1/2" and 7/16" mills have a 1/2" shank.
This is important to know, since you will need the appropriate sized collet or end mill
adaptor to match the shank size of the mill. The mini-mill comes with 3/8" and
1/2" collets, but you will eventually need 5/8" and 3/4" collets and/or end
mill adaptors to hold mills larger than 1/2".

2-Flute Versus 4-Flute

2-Flute mills are generally used for cutting slots or
grooves, while 4-flute mills are more often used for surface milling, using the end of the
mill. In practice, I have found that 4-flute mills will do most of what of a 2-flute
mill can do, but not vice-versa. But each type is best for certain operations.

What is the difference between 4 flute and 2
flute end mills? Is one better for Aluminum?

A 2 flute cutter (or slot mill) is designed
for "plunge" milling, ie you can plunge the mill directly into the work.
If you look at the business end of the cutter, you will see that the two blades are of
unequal length, one of the blades goes right across the middle. This allows the cutter to
mill directly under it. It can be used to cut vertically or horizontally, but the
horizontal cut is not likely to be as good as the 4 flute cutter. The 4 flute cutter (or
end mill)is not designed to be "plunged". Again, look at the cutters and you
will find that they are all the same length, but there is a hole in the middle where
nothing can be milled. This type of cutter can only be used to mill in a horizontal
direction. Graham

Um... That's (sort of) the difference between center cutting and non-center cutting, but
not really between 2 and 4 flute end mills. Some 4 flute end mills are center cutting
(usually two flutes meet in the middle and two don't). And, though I've never seen one,
there's no reason a 2 flute end mill couldn't be non-center cutting. (Never
saw a end mill with one "blade" longer than the other either... Not sure what
that's about.) The real difference has more to do with feeds, finish and such. Bascially a
cutters RPM is based on its composition and what you're cutting with it. So how much is
taken off per flute is, more or less, a "fixed" rate. How fast you can take off
how much material is a function of how many flutes you have. The fewer flutes, the slower
the feed must be. The more flutes, the faster the feeds can be. Also... I read a bit a
while back that explained how 2 flute end mills were better for slotting and 4 flute end
mills were better for finishing. The former had to do with the fact that the 2 flute
cutter bites into both walls at the same time and, when cutting to the front, cannot cut
the sides. Conversely a 4 flute end mill will cut both the front and sides at the same
time and the front cut will cause the cutter to flex, varying the depth of the side cuts
making the finish rougher. Don't claim to understand why a 4 flute cutter would give a
beter finish unless it's simply because it's stiffer...? As for aluminum... Though
"regular" end mills will work with aluminum, there are end mills specialy made
for aluminum. "High helix", I think they're called. They claim to produce a
better finish. Haven't messed with them much myself. But I'm heading that way for some
finishing work. I've only ever seen 2 flute versions of them. I'd love to find some 4
flute versions so I could increase my feeds...

Single-end Versus Double End

Double end mills are somewhat less expensive than single end
since you get two cutting ends on the same tool. I prefer single end mills since one end
of a double end mill sometimes becomes dull or damaged, but you lose track of that fact,
since you are keeping it around because of the 'good' end. Then you end up using the 'bad'
end by mistake and messing up your project. I suppose I could grind off the 'bad'
end to avoid this scenario. Another reason I like single-end mills is that the mill
sometimes becomes jammed in an end-mill adaptor and must be driven out using a soft metal
rod. This operation is likely to damage the top end of a double-end mill.